Microwave applicator with time-sharing of magnetron sources

ABSTRACT

A multimode microwave applicator or single-mode transmission line is excited by two microwave sources including magnetron oscillators. The sources are excited by full wave rectifier circuits, and in a manner so as to generate power only during portions of a cycle of the ac power supply. The respective rectifier circuits are energized by ac power sources that are displaced 90* in phase so that the sources operate in a timeshared mode to alternately transmit power to the applicator or line.

United States Patent 11 1 Johnson 1 Mar. 27, 1973 541 MICROWAVE APPLICATOR WITH 2,658,148 11/1953 Evans et al. ..33l/56 TIME SHARING 0F MAGNETRON 2,852,678 9/1958 Crapuchettes ..33l/55 SOURCES Inventor: Ray M. Johnson, Danville, Calif.

Assignee: Microdry Corporation, San Ramon,

Calif.

Filed: Oct. 29, 1971 Appl. No.: 193,902

US. Cl. ..331/5S, 219/1055, 331/86, 331/87 Int. Cl. ..H03b 9/10 Field of Search ...33l/55, 56, 86, 87; 315/3951, 315/38; 219/1055 Spencer ..331/56 Primary Examiner.lohn Kominski Attorney-Carl C. Batz [57] ABSTRACT A multimode microwave applicator or single-mode transmission line is excited by two microwave sources including magnetron oscillators. The sources are excited by full wave rectifier circuits, and in a manner so as to generate power only during portions of a cycle of the ac power supply. The respective rectifier circuits are energized by ac power sources that are displaced 90 in phase so that the sources operate in a time-shared mode to alternately transmit power to the applicator or line.

6 Claims, 4 Drawing Figures MICROWAVE APPLICATOR WITH TIlVlE- SHARING OF MAGNETRON SOURCES BACKGROUND OF THE INVENTION The present invention relates to a system for exciu'ng a multimode applicator or a single-mode transmission line to increase the average power in the applicator or line.

A multimode microwave applicator is a cavity in which microwaves, usually of a single frequency, are excited in a number of different modes to distribute the energy throughout the applicator for use in treating a material, for example, in heating wallboard during manufacture or in cooking food products or the like. A transmission line, of course, is simply a medium, such as a waveguide, for transmitting microwave power.

In the design of microwave heating applicators in the form of multimode cavities and in single-mode transmission line systems (herein generically referred to as applicators), it is sometimes desirable to increase the average input power into an applicator above the average power output of a single commercially available magnetron oscillator. It will be appreciated that a magnetron oscillator is extremely complex in design, so that an applications engineer normally starts with knowledge of existing magnetron sources, and designs the remainder of the system around the source. A magnetron oscillator, together with its power supply, is therefore normally obtained from a commercial source and it is usually the most expensive single item in a microwave system. These commercially available magnetron oscillators are rated for an average output power which cannot be exceeded without threatening damage to the magnetron tube.

One method of increasing the average power into any applicator would be to excite it with more than one between magnetrons is, to some extent, a function of I the design of the oscillator, but in general, the crosscoupled power has an adverse effect similar to that which occurs for high reflected power-namely, the energy couples into the interaction space and into the electron stream. The average power coupled into a magnetron depends upon the coupling coefiicient and the number of other magnetrons exciting a given applicator. Thus, a system which employs a number of magnetrons to excite a common multimode cavity (wherein the cross-coupling is small) or which employs even two magnetrons to excite a common transmission line (where the cross-coupling is relatively high)'has inherent disadvantages.

SUMMARY The present invention takes advantage of the fact that the instantaneous power of a magnetron oscillator may be increased without exceeding the average power rating for the tube as long as the duty cycle of the tube is correspondingly decreased. The duty cycle is the time when the oscillator is generating an output power. Thus, the instantaneous power level is increased when the tube is on, but the period between adjacent on cycles is also increased so that the average output power generated by the tube over an extended period of time does not exceed the average power rating for the tube.

Two or more magnetron oscillators are then energized by their associated power supplies in mutually exclusive relationship so that the output power of each tube is coupled into a common applicator in timeshared relationship.

In the illustrated embodiment, the timing for the two magnetron oscillators is developed from a three-phase HZ. voltage, four-wire system of the type generally available in industrial applications. Each magnetron oscillator is powered by a full wave rectifier circuit, and it turns on once during each half-cycle of the 60 HZ. supply. Circuitry is included which shifts the phase of the 60 HZ. power to one magnetron oscillator by Thus, the two oscillators are operated to generate power mutually exclusively, and this power is coupled to a common transmission line or applicator into which the combined power is coupled in time-shared relation.

Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of one embodiment of the invention accompanied by the attached drawing.

THE DRAWING FIG. 1 is a partially schematic, partially diagrammatic illustration of a system for exciting a transmission line with time-shared microwave energy from two separate magnetron oscillators;

FIG. 2 is an idealized graph showing the power output of a single magnetron oscillator;

FIG. 3 is an idealized graph showing the time relationship between the output of a single oscillator and its associated 6O HZ. supply voltage; and

FIG. 4 is a timing diagram showing the relationship between the combined power output of two magnetron oscillators operating in time-shared relation together with their associated 60 HZ. supply voltages.

DETAILED DESCRIPTION In order to more fully understand the present invention, a brief description will be given of the relevant characteristics of a particular commercially available magnetron oscillator. The oscillator to be described in an S-band magnetron oscillator normally used as a microwave power supply and manufactured by Litton Industries of Los Angeles, California, under the designation magnetron L-3858. An S-band oscillator operates in a frequency band between 2.4 GHZ. and 2.5 GHZ. This system includes a power supply for the oscillator tube employing a series field anode voltage which is derived from a full-wave rectifier energized by a normally available 60 HZ. voltage source, such as is available in a wall socket. The magnetron oscillator is not excited into oscillation until the output of the fullwave rectifier exceeds a design threshold voltage. Once this design threshold voltage is exceeded, oscillations within the magnetron build up, and the output power increases to a maximum occurring at the time the 60 HZ. power supply reaches a maximum, and the output power of the magnetron oscillator then decreases to zero.

Such an output envelope is diagrammatically illustrated in FIG. 2 wherein the abscissa is time in milliseconds and the ordinate is the output power of the magnetron. The bell-shaped cusps designated by reference numeral are idealized wave forms I representative of the envelope of the output power of the magnetron. That is, the magnetron oscillator is driven into oscillation beginning at time 10 for the first envelope 10 and the oscillations build up until the time t1 which also is the time at which the 60 HZ. supply reaches a peak. The actual oscillating voltage of the magnetron is, of course, at such a high frequency that it would not be visible on the time scale illustrated in FIG. 2. It will be observed that the cusps 10 are displaced from each other in time. The time between adjacent cusps (that is, the time between the end of one cusp and the beginning of a subsequent cusp) as compared with the on time of the oscillator determines the duty cycle.

The output power peak of the magnetron oscillator may be increased by increasing the peak-to-peak voltage of the 60 HZ. supply, and the duty cycle may be reduced by increasing the threshold voltage at which themagnetron is excited to oscillation. The average power rating for the tube is illustrated by the horizontal dashed line 1 1. By decreasing the duty cycle (that is, increasing the off time between adjacent cusps 10), and increasing the peak power output, indicated by the horizontal chain line 12 the average power output of the magnetron oscillator may be kept within the average rating while increasing the peak power output.

Turning now to FIG. 3, the cusps representing the envelope of the output power of a magnetron are again designated by reference numeral 10, and they are shown in solid line as a series of periodically occurring bell-shaped waveforms. FIG. 3 illustrates the time relationship of these power cusps with the 60 HZ. voltage input to the magnetron power supply which is represented by the dashed line 13 It will thus be appreciated that when the 60 HZ. voltage input reaches a predetermined threshold, the magnetron oscillator will be excited to generate output power, the voltage threshold being diagrammatically. represented by the horizontal chain line V Since the magnetron oscillator is powered by a full-wave rectifier, it is excited to oscillation once each half cycle of the 60 HZ. voltage input. Thus, output power of the magnetron bears a predetermined phase relationship with the input supply voltage. I

Referring now to FIG. 1, there is schematically illustrated a system for exciting a common transmission line with two separate magnetrons wherein the output power of the magnetrons are transmitted to the transmission line in mutually exclusive relation. The excitation of a transmission line according to the present invention is considered to be a more difficult problem than the excitation of a multimode cavity because of the higher cross-coupling between sources exciting a common transmission line. In FIG. 1, then, reference numeral 15 generally designates a first magnetron source and its associated launcher. Reference numeral 16 generally designates a second magnetron source and its associated launcher. Reference numeral 17 generally designates an H-plane waveguide tee having two input ports schematically designated by the dashed lines 18 and 19 respectively and an output port designated 20. The output port 20 feeds a waveguide transmission line schematically designated at 21.

The first magnetron source feeds port 18 by means of a first input waveguide 22; and the magnetron source 16 feeds the second input port 19 of the H-plane tee via a second input waveguide 23.

Turning now to the lower right-hand corner of FIG. 1, there is schematically designated the four terminals of a four-wire, three-phase 60 HZ. power supply, the line-to-line rrns voltage being 120 volts nominally. The magnetrons 15 and 16 are designed to be excited by a nominal voltage of 120 volts. The power supply is generally designated by reference numeral 25 and the four line terminals are designated respectively A, B, and C. The ground terminal is designated 0.

In terms of vector or phasor notation, the line voltage V is designated by the vector V Similarly, the other line voltages are designated according to standard notation. Further, the line-to-ground voltages are designated according to standard notation, for example, the voltage from line B to ground is designated V80.

It can be seen from the diagram that the line voltage V is out of phase with the line-to-neutral voltage V the latter voltage lagging the former voltage by 90. The line-to-line voltage V is connected by means of line 28, 29 to excite the magnetron oscillator 15. The line-to-neutral voltage Y is connected by means of lines 30, 31 to the primary winding 32 of a transformer generally designated by reference numeral 33. The transformer 33 has a secondary winding 34 having its output terminals connected by means of wires 35, 36 to energize the second magnetron oscillator 16. The transformer 33 is a step-up transformer having a ratio of 111.732 so that the voltage at the output terminals of secondary winding 34 has the same amplitude of the voltage energizing the magnetron oscillator 15; however, it lags in phase by 90, as indicated.

Referring now to FIG. 4, there is shown the superposition of the input voltages and the output power envelopes for the two magnetrons 15, 16 in FIG. 1 which are excited by 60 HZ. voltages shifted in phase relative to each other by 90. The sinusoidal voltage waveform 40 represents voltage V which energizes the first magnetron oscillator 15. Thus, the power envelope of the output power of the magnetron oscillator 15 is represented by the series of periodically spaced cusps 41 shown in dashed line. The voltage output of the transformer 33 is shown in dashed line and designated by reference numeral 42. The envelope of the output power of the magnetron l6 energized by the 60 HZ. voltage 42 is then illustrated by the series of periodically spaced cusps 44; and these are shown in solid line.

It will be recalled that the voltage output of the transformer 33 lags the voltage V by 90. The alternate power cusps 41, 44 are seen from FIG. 4 to occur in mutually exclusive relation. The average power from each individual magnetron has not increased beyond its rating; but the total power from both magnetrons has doubled while staying within its rated average power.

The power generated by magnetron oscillator 15 is transmitted through the transmission line 22 and into the port 18 of the H plane 17 from which it is coupled to the output transmission line 21. Similarly, the power output of the magnetron 16 is transmitted via waveguide 23 to the input port 19 of the H plane tee'17 from which it is coupled to the transmission line 21. It will be appreciated that a multimode cavity microwave applicator could be excited from the transmission line 21. Alternatively, the multimode cavity could be excited at two different locations by the magnetrons.

ln matching impedances at the input ports of H plane tee 17, it will be observed that when the magnetron energizing the port 18 is off (i.e., not oscillating), the input impedance looking from the port 18 toward the magnetron is pure reactance. The input impedance of the network as viewed from the port 19 can be matched to the output impedance of magnetron 16 by adjusting the reactance of port 18 to compensate for the reactance of the tee junction. This reactance is adjusted by varying the length of the transmission line 22, designated 1,. Similarly, when the magnetron 16 is not oscillating the magnetron 15 is on, the length of the transmission line 23 designated 1 is adjusted to compensate for the'reactive impedance of the H-plane tee 17 as viewed from the input port 18 so as to match the input impedance of that network to the output impedance of the magnetron 15. This impedance match should extend over the range of oscillation frequency possible for both of the magnetrons l5, 16. Hence, using a network of the type described in conjunction with mutually exclusively occuring output power envelopes of two magnetron oscillators, the effective sum of the output power envelopes from the two magment of the invention, persons skilled in the art will be able to substitute equivalent elements for those which have been disclosed and to modify certain aspects of this specific teaching while continuing to practice the invention; and it is, therefore, intended that all such modifications and substitutions be covered as they are embraced within the spirit and scope of the appended claims.

I claim:

1. A microwave system for energizing a common load com prising: first and second magnetron oscillator means; conductor means coupling the output of each of said magnetrons to said common load; a first source of voltage energizing one of said magnetron oscillators in a given phase to generate a power envelope for the output power of said first magnetron oscillator in phase relation with the input voltage thereof; and a second source of voltage connected to excite said second oscillator means, said second source of voltage being displaced in phase by 90 from said first source of voltage whereby said first and second magnetron oscillators are excited to generated output power envelopes occurring first conductive means connecting one line-to-line phase of a three-phase, four-wire supply voltage to one of said magnetron oscillators to energize the same; a

transformer having a primarg windin connected to said first terminal and a secon ary wm mg; means connecting an orthogonal line-to-neutral voltage of said four-wire supply to one winding of said transformer; and connective means for connecting the secondary winding of said transformer to energize said second magnetron oscillator means, said transformer having a step-up winding ratio such that the output voltage of the secondary thereof is equal in amplitude to the lineto-line voltage energizing said first oscillator means, said supply voltages to said first and second magnetron oscillator means being displaced in phase by 90.

3. A system for exciting a multimode cavity comprising first and second magnetron oscillator means, each oscillator means being energized by a 60 HZ. supply voltage and generating output power in predetermined phase relation with its associated power supply and only for a predetermined time during each cycle thereof, the average power of each oscillator means being within its design rating; and means for supplying each of said first and second oscillator means with a 60 HZ. supply voltage in 90 phase relation relative to each other.

4. The system of claim 7 wherein said supply means comprises a three-phase four-wire supply voltage having three power terminals and a neutral or ground terminal; means connecting one line-to-line phase of said supply voltage to one of said oscillator means; and means for connecting a line-to-neutral voltage of said supply voltage in orthogonal relation with said line-to- I line voltage thereof for supplying power to the second magnetron oscillator means.

5. The system of claim 8 further comprising transformer means connected in circuit with said line-toneutral supply voltage for stepping up the amplitude thereof to be equal to the amplitude of said line-to-line voltage. v

6. A microwave system for energizing a common load comprising: first and second magnetron oscillator means; conductor means coupling the output of each of said magnetrons to said common load; a first source of voltage energizing one of said magnetron oscillators in a given phase to generate a power envelope for the output power of said first magnetron oscillator in phase relation with the input voltage thereof; and a second source of voltage connected to excite said second oscillator means, said second source of voltage being displaced in phase from said first source of voltage whereby said first and second magnetron oscillators are excited to generated output power envelope occurring in alternation and in mutually exclusive relation, said first and second voltage sources comprising a threephase, four wire source of voltage, one of the line to line voltages supplying one of said magnetron oscillator means and an orthogonal line-to-neutral voltage and a step-up transformer supplying the other of said magnetron oscillator means. 

1. A microwave system for energizing a common load comprising: first and second magnetron oscillator means; conductor means coupling the output of each of said magnetrons to said common load; a first source of voltage energizing one of said magnetron oscillators in a given phase to generate a power envelope for the output power of said first magnetron oscillator in phase relation with the input voltage thereof; and a second source of voltage connected to excite said second oscillator means, said second source of voltage being displaced in phase by 90* from said first source of voltage whereby said first and second magnetron oscillators are excited to generated output power envelopes occurring in alternation and in mutually exclusive relation.
 2. In a system for exciting first and second magnetron oscillator means, each oscillator means being adapted to be energized by a 60 HZ. supply voltage to generate output power only when the input supply voltage exceeds a predetermined threshold thereby to generate an output power envelope in phase relation with the 60 HZ. input supply voltage and only during a portion of each half cycle thereof, the combination comprising first conductive means connecting one line-to-line phase of a three-phase, four-wire supply voltage to one of said magnetron oscillators to energize the same; a transformer having a primary winding connected to said first terminal and a secondary winding; means connecting an orthogonal line-to-neutral voltage of said four-wire supply to one winding of said transformer; and connective means for connecting the secondary winding of said transformer to energize said second magnetron oscillator means, said transformer having a step-up winding ratio such that the output voltage of the secondary thereof is equal in amplitude to the line-to-line voltage energizing said first oscillator means, said supply voltages to said first and second magnetron oscillator means being displaced in phase by 90*.
 3. A system for exciting a multimode cavity comprising first and second magnetron oscillator means, each oscillator means being energized by a 60 HZ. supply voltage and generating output power in predetermined phase relation with its associated power supply and only for a predetermined time during each cycle thereof, the average power of each oscillator means being within its design rating; and means for supplying each of said first and second oscillator means with a 60 HZ. supply voltage in 90* phase relation relative to each other.
 4. The system of claim 7 wherein said supply means comprises a three-phase four-wire supply voltage having three power terminals and a neutral or ground terminal; means connecting one line-to-line phase of said supply voltage to one of said oscillator means; and means for connecting a line-to-neutral voltage of said supply voltage in orthogonal relation with said line-to-line voltage thereof for supplying power to the second magnetron oscillator means.
 5. The system of claim 8 further comprising transformer means connected in circuit with said line-to-neutral supply voltage for stepping up the amplitude thereof to be equal to the amplitude of said line-to-line voltage.
 6. A microwave system for energizing a common load comprising: first and second magnetron oscillator means; conductor means coupling the output of each of said magnetrons to said common load; a first source of voltage energizing one oF said magnetron oscillators in a given phase to generate a power envelope for the output power of said first magnetron oscillator in phase relation with the input voltage thereof; and a second source of voltage connected to excite said second oscillator means, said second source of voltage being displaced in phase from said first source of voltage whereby said first and second magnetron oscillators are excited to generated output power envelope occurring in alternation and in mutually exclusive relation, said first and second voltage sources comprising a three-phase, four wire source of voltage, one of the line to line voltages supplying one of said magnetron oscillator means and an orthogonal line-to-neutral voltage and a step-up transformer supplying the other of said magnetron oscillator means. 